1. Field of the Invention
The present invention relates to a fuel vapor emission control device for an engine.
2. Description of the Related Art
In a general engine for, for example, a motor vehicle, fuel in a fuel tank is supplied by a fuel pump to a fuel injector, and is fed into the engine by the fuel injector. When the fuel consumption is small, such as in an idling operation of the engine, a surplus of fuel fed to the injector is returned to the fuel tank. When the surplus is flowing through piping, it may be heated by high temperature parts, such as a cylinder head of the engine around the piping, and thus, fuel of high temperature may be returned to the fuel tank. On the other hand, the temperature of fuel in the fuel tank is also increased by ambient temperature when the engine is stopped, if the ambient temperature is high. If the temperature of the fuel in the fuel tank is increased, a large amount of fuel vapor may be generated in the fuel tank to thereby increase the pressure in the fuel tank. At this state, the fuel tank may deform. To reduce the pressure in the fuel tank, the fuel vapor is discharged out of the fuel tank.
However, it is not desirable to discharge fuel vapor into the outside air. To prevent the fuel vapor from escaping to the outside air, motor vehicles have been provided with an adsorbing layer made of, for example, activated charcoal to trap fuel vapor therein. Fuel vapor adsorbed in the activated charcoal is released by air passing through the activated charcoal. The released fuel component and air are fed into an intake passage of the engine and are burned in a combustion chamber together with injected fuel.
Such a kind of fuel vapor emission control device is disclosed in, for example, Japanese Unexamined Patent publication No. 61-53451. In JPP '451, all of fuel vapor generated in the fuel tank is, first, introduced into the activated charcoal layer. After being temporarily adsorbed in the layer, the fuel vapor is purged into the intake passage of the engine, to prevent the amount of fuel vapor purged into the intake passage of the engine becoming too large, so that the fluctuation in the air-fuel ratio during the purging operation becomes smaller.
The activated charcoal used for the adsorbent deteriorates after repeated adsorption and release of fuel vapor. Therefore, if a large amount of fuel vapor from the fuel tank is always introduced into the activated charcoal, as in the device mentioned above, the activated charcoal rapidly deteriorates. If the activated charcoal deteriorates, fuel vapor is purged into the intake passage without temporarily being adsorbed in the activated charcoal. A problem occurs that the large amount of fuel vapor may be purged into the intake passage of the engine, and thereby, it causes an undesirable wide fluctuation in the air-fuel ratio.